1. Field of the Invention
The present invention relates to a method for forming an embedded circuit. In particular, the present invention relates to a method for forming an embedded circuit by using a dummy layer. The dummy layer is not only resistant to the acidic or alkaline conditions or organic solvents, but also does not interfere with the formation of the seed layer. The method of the present invention is advantageous in mass production and in circuit board structure.
2. Description of the Prior Art
A circuit board is an essential element in an electronic device. The function of the circuit boards is to define the pre-determined circuit patterns on a solid surface. Along with the trend of miniaturization of the electronic devices, the line width and the line space of the conductive wires on the circuit boards are becoming narrower and narrower. The embedded circuit structure draws more and more attention than ever in order to pursue a thinner product, to meet the demands of finer wires and to overcome the drawbacks of the etching procedure and reliability. Because in the embedded circuit structure the wire pattern is embedded in the substrate, the thickness of the wires seems to be omitted, thereby further reducing the thickness of the products after packaging.
With the development of shrinking dimensions of the electronic devices, the circuit board is usually disposed in a limited space. Currently, there are several methods available to form the circuit boards to meet the demand. The first one is to transfer-print patterned wires into a dielectric layer. Another one is to pattern the substrate by means of a laser fashion to define an intaglio structure. Then a conductive material is used to fill the recess formed on the substrate to obtain an embedded circuit structure.
For the current solution, it is produced by direct circuit design. For example, a laser is used to pattern a substrate. Then a conductive material is used to fill the recess formed on the substrate to obtain an embedded circuit structure. FIGS. 1-5 illustrates a conventional method to form an embedded circuit. Please refer to FIG. 1, first a substrate 101 is provided. The substrate 101 includes a dielectric layer 110, an inner layer 111 and an interconnecting circuit 112. The interconnecting circuit 112 is disposed on the inner layer 111 and the dielectric layer 110 covers the interconnecting circuit 112 and the inner layer 111 at the same time. In addition, the dielectric layer 110 also includes a via column 113 which is electrically connected to the interconnecting circuit 112.
Second, as shown in FIG. 2, a mask layer 114 is used to completely cover the dielectric layer 110 and the via column 113. The mask layer 114 is required to have many features to facilitate the following steps. The features of the mask layer 114 will be described in the following paragraphs.
First, as shown in FIG. 3, a laser is used to pattern the mask layer 114. The laser may be used to define the pattern and the location of the needed circuits. For example, trenches 115 of different width are formed after the laser patterns the mask layer 114. Some of the trenches 115 may expose the via column 113.
Then, as shown in FIG. 4, a desmear step is carried out. Because there may be some remaining residues 116 after the laser patterns the mask layer 114 and the remaining residues 116 degrade the quality of the following electrical connection, a pre-treatment step is carried out to remove the remaining possible residues 116 after the laser patterns the mask layer 114 to facilitate the later formation of the electrical connection. There may be an organic solvent or an oxidizing agent used in the pre-treatment step so the mask layer 114 must be resist to the corrosion of the organic solvent or the oxidizing agent. In addition, a base or an acid may also be used in the pre-treatment step, so the mask layer 114 must also be resistant to the corrosion of the acid or the base.
Next, as shown in FIG. 5, a seed layer 117 is formed. The resultant seed layer 117 may induce and facilitate the formation of the later formed copper circuit (not shown) in the trench 115. Because the copper circuit (not shown) is required solely and exclusively to form in the trench 115, the seed layer 117 accordingly has to be selectively and specifically formed in the trench 115 without covering the mask layer 114. By the difference between the mask layer 114 and the dielectric layer 110, the seed layer 117 is required to solely and exclusively form on the exposed dielectric layer 110 rather than on the mask layer 114.
In the light of the above, the mask layer 114 not only must be resistant to the corrosion of the organic solvent or the oxidizing agent, but also be resistant to the corrosion of the acid or the base and impossible for the formation of the seed layer 117 so the seed layer 117 is selectively and specifically formed in the trench 115. The development of the materials for the mask layer 114 becomes very difficult due to the various requirements of the mask layer 114.
Further, in order to avoid the mask layer 114 falling off due to the attack of the chemical agents in the desmear step, weaker chemical agents are used. However, when the chemical agents are weaker, it is more likely to have remaining residues and the reliability of the products is more likely to be compromised. Circuits of bad quality are not welcome.
Given the above, a novel method for forming an embedded circuit is still needed to provide a reliable circuit board product.